The present invention relates a system and method for operating a fuel cell system and, more particularly, to a system and method for controlling the relative humidity or dew point of a fuel cell system's cathode inflow.
Fuel cells are used as a power source for electric vehicles, stationary power supplies and other applications. One known fuel cell is the Proton Exchange Membrane (“PEM”) fuel cell that includes a plurality of membrane-electrode-assemblies (“MEAs”). A MEA comprises a thin, solid polymer membrane-electrolyte having an anode on one face and a cathode on the opposite face and is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode. The collectors typically contain appropriate channels and openings for distributing the fuel cell's gaseous reactants (e.g., hydrogen/H2 and oxygen/O2) over the surfaces of the respective anode and cathode.
PEM fuel cells comprise a plurality of the MEAs in electrical series (collectively referred to as a stack) while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate or current collector.
The fuel cells are operated in a manner that maintains the MEAs in a humidified state. The cathode and/or anode reactant gases being supplied to the fuel cell are typically humidified to prevent drying the MEAs in the locations proximate the reactant gases inlets. The level of the MEAs' humidity affects the performance of the fuel cell. Additionally, if an MEA is run too dry, the MEA can be damaged which can cause immediate failure or reduce the useful life of the fuel cell.
The operation of the fuel cells with the MEAs humidified too much, however, may also limit the fuel cell stack's performance. Specifically, the formation of liquid water can impede the diffusion of gas to the MEAs, thereby limiting their performance. Liquid water may also act as a flow blockage reducing cell flow and causing even higher fuel cell relative humidity which can lead to unstable fuel cell performance. Additionally, the formation of liquid water within a cell can cause significant damage when the fuel cell is shut down and exposed to freezing conditions. That is, when the fuel cell is non-operational and the temperature in the fuel cell drops below freezing, the liquid water therein will freeze and expand, potentially damaging the fuel cell.